The present invention relates to a process for preparing a curable coating and for curing thereof.
Oligomers, polymers with low Dp (degree of polymerization), of acrylate or methacrylate unit-containing backbones are of commercial interest and have industrial uses for many different applications, such as adhesives, inks, coatings, films, and others. A suitable low Dp value will provide a material with a molecular weight high enough for reduced toxicity, yet low enough for low viscosity. However, production of such oligomers has proven to be difficult and is frequently carried out by cumbersome and/or not very selective processes. It becomes even more difficult if a crosslinkable or curable oligomer composition is desired for the applications. This is because crosslinking or curing property typically requires the presence of additional reactive pendant groups in the oligomers. Such reactive pendant groups may be partially or substantially eliminated or reacted away by unintended side reactions or premature crosslinking reactions during the oligomerization reaction.
Several approaches have been tried and used to effect production of such oligomers. For example, one approach uses chain transfer agents to control Dp. As a result of the chain transfer chemistry involved, one chain transfer agent is incorporated into each backbone structure of the oligomers. This makes the oligomer property much less uniform and harder to control. In addition, the most commonly used chain transfer agents are mercaptans. Due to their odors and chemical properties, it becomes increasingly more difficult socially and less acceptable environmentally to use such sulfur-based materials. Other common chain transfer agents such as hypophosphites, bisulfites and alcohols would also impart additional functionalities into the oligomers. Such additional functionalities may not be compatible with other ingredients in a formulated product or suitable for the intended applications. Removal of the additional functionality from the resultant oligomers may be difficult and/or expensive.
Another approach calls for the use of large amounts of initiators or catalysts. This approach adds raw material cost to oligomer production. It also may result in undesirable oligomer chain degradations, branching, and unintended or premature crosslinking of the product prior to use. In addition, any residual initiators or catalysts in the product may have to be removed before the product can be used for many applications to avoid compatibility or contamination problems.
U.S. Pat. No. 4,356,288, discloses the preparation of terminally-unsaturated oligomers with a Dp in the range of from about 6 to about 30 from esters of acrylic acid by an anionic polymerization reaction carried out in the presence of a catalytic amount of an alkoxide anion. Alkoxide anions are known to be sensitive to water. Accordingly, the method is often adversely affected by the presence of moisture, resulting in lower yield and/or lower uniformity of the oligomer product.
Another patent, U.S. Pat. No. 5,710,227, discloses a high temperature, continuous polymerization process for preparing terminally unsaturated oligomers which are formed from acrylic acid and its salts, and acrylic acid and its salts with other ethylenically unsaturated monomers. The high temperature, continuous polymerization process solves many of the problems associated with previously known methods for preparing terminally-unsaturated oligomers formed from acrylic acid. However, the neat form of many of the acrylic acid products are solid at room temperature and, thus, requiring either heating and/or the addition of a solvent to handle and use the products.
U.S. Pat. No. 5,484,850 discloses copolymer compositions which are crosslinkable by a free radical method and have a Mn from 1500 to 6000 and a polydispersity of 1 to 4. Copolymer A is composed of from 50 to 85 mol % of a monomer (al) containing methacryloyl group; from 15 mol % to 50 mol % of another monomer (a2) capable of undergoing free-radical polymerization; and from 5 mol % to 50 mol % of the total amount of the monomers (a1) and (a2) being monomers (a3) which carry functional groups selected from the group consisting of hydroxy, carboxyamido, amino, carbonyl, isocyanate, carboxyl and epoxy, the functional groups being capable of undergoing a condensation or addition reaction. The polymerization is carried out at a temperature from 140 to 210xc2x0 C. and with an average residence time of from 2 minutes to 90 minutes. Copolymer A reacts with an olefinically unsaturated monomer B which carries a functional group which is complementary to the functional groups of monomers (a3). The products are solids which tend to limit their uses and processing options.
The present invention seeks to overcome the problems associated with the previously disclosed methods for preparing oligomers, particularly curable or crosslinkable liquid oligomers, by providing an oligomerization process that produces curable oligomers with a low Dp, in the range of from 3 to 100, without the need of excessive amounts of initiators. The curable oligomer products are in liquid form and may be terminally unsaturated. The crosslinkable or curable functionality is incorporated into the oligomer by a reaction after the oligomerizationxe2x80x94a post-oligomerization reactionxe2x80x94between the oligomer or altered oligomer with a modifier which contains a crosslinkable/curable functional group. The present invention also provides curable oligomer compositions prepared according to the disclosed process. Furthermore, the invention provides curable oligomer compositions which are substantially free of metals, salts and/or surfactant contaminants. The product from the present invention is useful for a number of applications, such as films, markings, coatings, paints, adhesives, binders, inks and others.
One aspect of the present invention relates to a method for producing a curable coating on a substrate surface comprising:
(A) applying to said substrate surface a layer of a curable composition; and
(B) drying, or allowing to dry, said layer,
wherein said curable composition is a composition prepared by a process comprising the steps of:
(a) forming an oligomer having a Dp in the range of from 3 to 100 by either:
(i) oligomerizing a mixture comprising a monomer A and a monomer B, wherein said monomer A has at least one functional group which remains substantially unreacted during said oligomerizing; or
(ii) oligomerizing a mixture comprising a monomer A equivalent and a monomer B and generating said at least one functional group during or after said oligomerizing;
wherein said oligomer has a first number of monomer units incorporated into its backbone; and
wherein said oligomerizing is conducted at a temperature in the range of from 150xc2x0 C. to 650xc2x0 C., a pressure in the range of from 3 MPa to 35 MPa which is sufficient to maintain said mixture in a fluid state, and a residence time in the range of from 0.1 second to 1 minute; and
(b) reacting a modifier having at least one reactive moiety with said oligomer through a reaction under a second condition between said reactive moiety of said modifier and said functional group incorporated into said oligomer to produce said curable composition, wherein said modifier further comprises a curable group selected from the group consisting of a carbonxe2x80x94carbon double bond, an oxygen-containing heterocyclic group and mixtures thereof, and said curable group remains pendant in said curable composition and crosslinkable after said reaction.
A second aspect of the present invention relates to a method for producing a curable coating on a substrate surface comprising:
(A) applying to said substrate surface a layer of a curable composition; and
(B) drying, or allowing to dry, said layer,
wherein said curable composition is a composition prepared by a process comprising the steps of:
(a) forming an oligomer having a Dp in the range of from 3 to 100 by either:
(i) oligomerizing a mixture comprising a monomer A and a monomer B, wherein said monomer A has at least one functional group which remains substantially unreacted during said oligomerizing; or
(ii) oligomerizing a mixture comprising a monomer A equivalent and a monomer B and generating said at least one functional group during or after said oligomerizing;
wherein said oligomer has a first number of monomer units incorporated into its backbone; and
wherein said oligomerizing is conducted at a temperature in the range of from 275xc2x0 C. to 450xc2x0 C., a pressure in the range of from 3 MPa to 35 MPa which is sufficient to maintain said mixture in a fluid state, and a residence time in the range of from 0.1 second to 4 minutes;
(b) reacting a modifier having at least one reactive moiety with said oligomer through a reaction under a second condition between said reactive moiety of said modifier and said functional group incorporated into said oligomer to produce said curable composition, wherein said modifier further comprises a curable group selected from the group consisting of a carbonxe2x80x94carbon double bond, an oxygen-containing heterocyclic group and mixtures thereof, and said curable group remains pendant in said curable composition and crosslinkable after said reaction.
A third aspect of the present invention relates to a method for producing a curable coating on a substrate surface comprising:
(A) emulsifying a curable composition in an aqueous medium, optionally in the presence of a surfactant, to form a waterborne formulation;
(B) applying to said substrate surface a layer of said waterborne formulation; and
(C) drying, or allowing to dry, said layer,
wherein said curable composition is a composition prepared by a process comprising the steps of:
(a) forming an oligomer having a Dp in the range of from 3 to 100 by either:
(i) oligomerizing a mixture comprising a monomer A and a monomer B, wherein said monomer A has at least one functional group which remains substantially unreacted during said oligomerizing; or
(ii) oligomerizing a mixture comprising a monomer A equivalent and a monomer B and generating said at least one functional group during or after said oligomerizing;
wherein said oligomer has a first number of monomer units incorporated into its backbone; and
wherein said oligomerizing is conducted at a temperature in the range of from 150xc2x0 C. to 650xc2x0 C., a pressure in the range of from 3 MPa to 35 MPa which is sufficient to maintain said mixture in a fluid state, and a residence time in the range of from 0.1 second to 1 minute; and
(b) reacting a modifier having at least one reactive moiety with said oligomer through a reaction under a second condition between said reactive moiety of said modifier and said functional group incorporated into said oligomer to produce said curable composition, wherein said modifier further comprises a curable group selected from the group consisting of a carbonxe2x80x94carbon double bond, an oxygen-containing heterocyclic group and mixtures thereof, and said curable group remains pendant in said curable composition and crosslinkable after said reaction.
A fourth aspect of the present invention relates to a method for producing a curable coating on a substrate surface comprising:
(A) emulsifying a curable composition in an aqueous medium, optionally in the presence of a surfactant, to form a waterborne formulation;
(B) applying to said substrate surface a layer of said waterborne formulation; and
(C)drying, or allowing to dry, said layer,
wherein said curable composition is a composition prepared by a process comprising the steps of:
(a) forming an oligomer having a Dp in the range of from 3 to 100 by either:
(i) oligomerizing a mixture comprising a monomer A and a monomer B, wherein said monomer A has at least one functional group which remains substantially unreacted during said oligomerizing; or
(ii) oligomerizing a mixture comprising a monomer A equivalent and a monomer B and generating said at least one functional group during or after said oligomerizing;
wherein said oligomer has a first number of monomer units incorporated into its backbone; and
wherein said oligomerizing is conducted at a temperature in the range of from 275xc2x0 C. to 450xc2x0 C., a pressure in the range of from 3 MPa to 35 MPa which is sufficient to maintain said mixture in a fluid state, and a residence time in the range of from 0.1 second to 4 minutes;
(b) reacting a modifier having at least one reactive moiety with said oligomer through a reaction under a second condition between said reactive moiety of said modifier and said functional group incorporated into said oligomer to produce said curable composition, wherein said modifier further comprises a curable group selected from the group consisting of a carbonxe2x80x94carbon double bond, an oxygen-containing heterocyclic group and mixtures thereof, and said curable group remains pendant in said curable composition and crosslinkable after said reaction.
The term xe2x80x9coligomerxe2x80x9d used herein means a polymer composition which has a degree of polymerization (Dp) in the range of from 3 to 100. Unless otherwise specified in the present application, the term xe2x80x9cpolymerizationxe2x80x9d is used herein as a generic term and interchangeably with the term xe2x80x9coligomerization.xe2x80x9d An oligomer has a number of monomer units incorporated into the backbone. Dp is determined as a monomer unit average number. Depending on the oligomerization reaction mechanism, the actual number of carbon atoms in a particular oligomer backbone may be of an even or an odd number, even though the carbonxe2x80x94carbon double bonds in the monomers have two carbons each. Since it is rare that all of the oligomer molecules have the same total number of monomer units incorporated into the backbone, there is usually a distribution of various oligomers with either smaller and/or larger Dp than the range indicated and/or preferred in the application. This type of distribution is also known to exist in almost all polymers and it is commonly referred to as xe2x80x9cpolydispersity.xe2x80x9d A preferred Dp for this invention is in the range of from 5 to 50. A more preferred Dp is in the range of from 5 to 20.
The present invention also relates to a curable composition, particularly by UV, visible light, electron beam methods, prepared by a process comprising forming an oligomer with a Dp in the range of from 3 to 100 from oligomerization of a mixture which comprises a monomer A and a monomer B under a first condition, wherein the monomer A has at least one functional group which either is generated after the oligomerization or is present in the monomer A before the oligomerization and remains substantially unreacted during the oligomerization; wherein the monomer B is selected from the group consisting of ethylene, propylene, C4 to C10 xcex1-olefins, butadiene, isoprene, styrene, substituted styrene, vinyl ester, vinyl ether, vinyl silane, vinyl halide, acrylic acid, methacrylic acid, crotonic acid, alkyl acrylate ester, alkyl methacrylate ester, alkyl crotonate ester, acrylamide, methacrylamide, N-substituted acrylamide, N-substituted methacrylamide and mixtures thereof; the oligomer has a first number of monomer units incorporated into its backbone; and wherein the first condition comprises a temperature in the range of from 150xc2x0 C. to 650xc2x0 C. and a pressure in the range of from 3 MPa to 35 MPa which is sufficient to maintain the mixture in a fluid state, and a residence time at the temperature and the pressure in the range of from 0.1 second to 4 minutes; and reacting a modifier having at least one reactive moiety with the oligomer through a reaction under a second condition between the reactive moiety of the modifier and the functional group of the monomer A incorporated into the oligomer to produce the curable composition, wherein the modifier further comprises a curable group selected from the group consisting of a carbonxe2x80x94carbon double bond, an oxygen-containing heterocyclic group and mixtures thereof, and the curable group remains pendant in the curable composition and crosslinkable after the reaction.
The word xe2x80x9cpendantxe2x80x9d means that a group, a functional group or a reactive moiety, is not in the backbone structure itself of an oligomer or polymer. A reaction of a pendant group for the present invention will not cause any changes of the backbone structure itself. A pendant group, may be attached directly to a carbon in the backbone structure of the oligomer. Examples of this directly-attached type pendant groups include xe2x80x94OH or xe2x80x94OC(xe2x95x90O)(CH3) group (from vinyl acetate monomer); and xe2x80x94COOH or xe2x80x94COO(R) group (from acrylates or methacrylates). Or, there may be other intermediate chemical moieties or groups between the functional group and the carbon atom in the backbone structure to server as xe2x80x9clinkers.xe2x80x9d An example of this type is the xe2x80x94OH group in 2-hydroxyethyl methacrylate when it is used as one of the monomers. There is a xe2x80x94CH2CH2xe2x80x94 group between the xe2x80x94OH group and the xe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94 group of the backbone structure. Other linker examples include (xe2x80x94Oxe2x80x94CH2CH2xe2x80x94)n where n is in the range of 1 to 10. Others become clear from the rest of the description of the present invention. Pendant groups for this invention are generally reactive, either they are suitable for attaching a crosslinkable or curable group to the oligomers, or they are used for curing or crosslinking. Curing and crosslinking are used herein interchangeably.
To facilitate understanding of the present invention, a general scheme is summarized below. It is used for illustration purposes only, not intended for limiting the scope of invention which is defined herein by the specification and the claims. It is also understood that some of the steps may be carried out simultaneously or sequentially.
monomer A+monomer Bxe2x86x92oligomer[xe2x86x92post-oligomerization generation of functional groups, optional]xe2x86x92reaction with a modifier to form a curable composition[xe2x86x92emulsion formation to form a curable formulation, optional]xe2x86x92curing or crosslinking.
In the instant invention, an oligomer is prepared by an oligomerization reaction of a mixture which comprises a monomer A and a monomer B. The mixture may further comprise a solvent and other materials for a variety of purposes such as catalysis or reaction mediation. The monomer A and the monomer B preferably are different, but they may be the same in certain specific cases wherein the monomer A is produced by transforming the monomer B incorporated in an oligomer after the oligomerization reaction, as described below in more detail. The monomer A and the monomer B may be premixed, with or without a solvent, prior to the oligomerization, or they may be introduced separately into a reaction zone at a predetermined rate or manner. The latter typically requires a mechanism to provide proper mixing. The mechanism can be static such as specially designed inlets, nozzles, or mobile such as a mechanical stirring device. For the present invention, it is preferred to have the monomers and optionally a solvent, if present, premixed before they are fed into the reactor.
For the present invention, the monomer A must have, in addition to a polymerizable or oligomerizable carbonxe2x80x94carbon double bond, a functional group, which does not participate in, or remains pendant or substantially unreacted during the oligomerization reaction. Such a functional group may be already present in the monomer A itself prior to the oligomerization or it may be generated after the oligomerization from a xe2x80x9cmonomer A equivalent.xe2x80x9d After the functional group is generated, the reaction between the modifier through its reactive moiety and the functional group of the oligomer can be carried out to form the curable composition.
It is within the scope of the present invention to generate the functional group post-oligomerization from a xe2x80x9cmonomer A equivalentxe2x80x9d, i.e. after the oligomerization is completed or substantially completed. This requires the use of a xe2x80x9cmonomer A equivalentxe2x80x9d in the oligomerization reaction and at least one additional conversion reaction to generate the desired functional group. It is also possible to have the additional conversion reaction and the oligomerization occur almost simultaneously.
A xe2x80x9cmonomer A equivalentxe2x80x9d is an oligomerizable or polymerizable monomeric carbonxe2x80x94carbon-double-bond-containing compound which has another group that may be converted to produce the desired functional group after the oligomerization or polymerization is completed or substantially completed during the oligomerization reaction. A xe2x80x9cmonomer A equivalentxe2x80x9d may be the same as the monomer B used in the oligomerization reaction.
There may be various reasons and benefits for using a xe2x80x9cmonomer A equivalent.xe2x80x9d For example, vinyl alcohol does not have a chemically stable monomeric form which can be easily used in an oligomerization or polymerization reaction. Accordingly, vinyl acetate is used most frequently as vinyl alcohol""s xe2x80x9cequivalentxe2x80x9d and the acetate group is converted by hydrolysis to generate the desired hydroxy (OH) group after the oligomerization or polymerization reaction is completed. If desired, the acetate group can also be converted into an acrylate or methacrylate group via a trans-esterification reaction or by hydrolysis followed by direct esterification.
Following is another example of this type of post-oligomerization generation of functional groups, particular pendant functional groups, where the xe2x80x9cmonomer A equivalentxe2x80x9d is the same as the monomer B. For instance, a homo-oligomer of methyl acrylate may be partially or completely hydrolyzed to form carboxylic acid groups, i.e. xe2x80x94COOMe groups are transformed into xe2x80x94COOH functional groups via hydrolysis. Such functional groups then can be reacted with a modifier having a reactive moiety such as glycidyl (meth)acrylate or hydroxyalkyl ester of acrylic or methacrylic acid to achieve the desired incorporation of crosslinkable or curable carbonxe2x80x94carbon double bonds. Another example involves a co-oligomer prepared from different esters of ethylenically unsaturated acids. A typical co-oligomer may be made from methyl acrylate and n-butyl methacrylate. A post-oligomerization hydrolysis reaction will also produce xe2x80x94COOH groups. One advantage of such a post-oligomerization generation of functional groups is that the hydrolysis reaction can be controlled or adjusted to give the desired level or amount of functional groups in the oligomer products. Because different monomer units in the oligomer structure usually have different hydrolysis or trans-esterification rates, this method provides another way of controlling the incorporation of crosslinkable functionalities. Another type of post-oligomerization generation of functional groups involves hydrolysis of pendant amide groups.
Many compounds are suitable for use as the monomer A in the present invention. The selection depends primarily on the monomer B used and the reactive moiety on the selected modifier. The monomer A must have at least one functional group that exists after or does not substantially participate in the oligomerization reaction. General categories of such functional groups include carbonxe2x80x94carbon double bond, halide, hydroxyalkyl, hydroxyaryl, carboxylic acid or ester, epoxy (or oxiranyl), oxetanyl, anhydride, alkylsiloxy, alkoxysilyl, and arylsiloxy groups. Groups like anhydride could be in a form incorporated through the carbonxe2x80x94carbon double bond of monomers like maleic anhydride, citraconic anhydride, and itaconic anhydride. It is understood that not all the disclosed functional groups can be used for all the different types of oligomerization reactions. It is also understood that not all the functional groups will react with all the reactive moieties of all modifiers. For the present invention, there must be a reasonable reaction rate between the functional group and the reactive moiety under a second reaction condition, with or without a catalyst or reaction mediator. Chemical compatibility must also be satisfied. Specific limitations on functional groups and reactive moieties are further disclosed herein.
Examples of a monomer A suitable for the present invention include: acrylic acid, methacrylic acid, 1,3-butadiene, isoprene, 4-vinylcyclohexene, allyl alcohol, allyl esters such as allyl acetate, allyl propionate, allyl acrylate, allyl methacrylate, allyl crotonate, vinyl acrylate, vinyl methacrylate, vinyl crotonate, vinyl chloride, vinyl bromide, vinylidene chloride, vinylidene fluoride, vinyl acetate, vinyl benzoate, norbornadiene, substituted norbornadienes, 4-vinylcyclohexene oxide, glycidyl methacrylate, glycidyl acrylate, glycidyl crotonate, 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, acrolein, methacrolein, maleic anhydride, itaconic anhydride, citraconic anhydride, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltrichlorosilane, allyltrichlorosilane, allyltrimethoxysilane, allyltriethoxysilane, xcex3-methacryloxypropyltrimethoxysilane, and mixtures thereof.
Examples of a xe2x80x9cmonomer A equivalentxe2x80x9d include vinyl acetate, vinyl halide (such as vinyl chloride, vinyl bromide, vinyl iodide, vinyl fluoride), vinylidene halide, allyl acetate, allyl propionate, methacrylonitrile, acrylonitrile, C1-C20 alkyl acrylate esters, C1-C20 alkyl methacrylate esters, C1-C20 alkyl crotonate esters, acrylamide and N-substituted acrylamides such as N-methylacrylamide, methacrylamide and N-substituted methacrylamides such as N,N-dimethylmethacrylamide, and mixtures thereof. The corresponding functional groups generated are xe2x80x94OH (vinyl acetate and allyl acetate) and xe2x80x94COOH (others) respectively. Depending on the desired products, certain monomer A such as maleic anhydride, itaconic anhydride and citraconic anhydride also could serve as a xe2x80x9cmonomer A equivalentxe2x80x9d to produce dicarboxylic acid functional groups.
The monomer B is typically an ethylenically unsaturated monomer and its derivatives thereof, such as olefins, styrenes, unsaturated carboxylic acids, esters and amides, vinyl esters, vinyl ethers, vinyl silanes, and mixtures thereof. A preferred monomer B comprises xcex1,xcex2-ethylenically unsaturated carboxylic acids, preferably acrylic acid and methacrylic acid, and their esters of linear or branched alcohols containing from 1 to 20 carbons. Specific examples of monomer B include, but are not necessarily limited to, ethylene, propylene, C4-C10 xcex1-olefins, 1,3-butadiene, isoprene, styrene, substituted styrenes such as p-methylstyrene, vinyl acetate, vinyl benzoate, vinyl chloride, vinyl bromide, allyl acetate, methyl acrylate, methyl methacrylate, methyl crotonate, ethyl acrylate, ethyl methacrylate, ethyl crotonate, n-propyl acrylate, n-propyl methacrylate, n-propyl crotonate, i-propyl acrylate, i-propyl methacrylate, i-propyl crotonate, n-butyl acrylate, n-butyl methacrylate, n-butyl crotonate, sec-butyl acrylate, sec-butyl methacrylate, sec-butyl crotonate, ethyl 4,4,4-trifluorocrotonate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-ethylhexyl crotonate, acrylic acid, methacrylic acid, crotonic acid, acrylamide, methacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N,N-dimethylmethacrylamide, N,N-diethylmethacrylamide, ethyl vinyl ether, n-propyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, 2-ethylhexylvinyl vinyl ether, 2-chloroethyl vinyl ether, 2-aminoisobutyl vinyl ether, vinyltrimethylsiane and mixtures thereof.
Where the desired functional groups are already present in the monomer A and remain substantially unreacted or pendant during and after the oligomerization, the molar ratio of the monomer A to monomer B incorporated into the backbone of the oligomer produced is in the range of from 1:40 to 40:1, preferably in the range of from 1:20 to 20:1, most preferably in the range of from 1:5 to 5:1.
Where the functional groups are generated post-oligomerization from the xe2x80x9cmonomer A equivalentxe2x80x9d incorporated into the oligomers, the ratio of the number of generated functional groups to the number of total monomer units in the oligomer backbone is in the range of from 1:100 to 1:1. It is also within the scope of the present invention if the xe2x80x9cmonomer A equivalentxe2x80x9d can be converted into more than one functional group, say z number of groups, per monomer unit, the ratio could exceed 1:1 to as high as z:1. For example, if a monomer has a maleic anhydride group which can be converted into two carboxylic functional group per monomer unit. The ratio could exceed 1:1 to 1.5:1 or to a maximum of 2:1.
The oligomerization reaction is carried out under a first condition which comprises a temperature of at least 150xc2x0 C., generally in the range from 150xc2x0 C. to 650xc2x0 C., preferably in the range of from 200xc2x0 C. to 500xc2x0 C., more preferably from 275xc2x0 C. to 450xc2x0 C. and a pressure in the range of from 3 MPa to 35 MPa, preferably in the range of from 20 MPa to 30 MPa. A preferred combination of temperature and pressure is in the ranges of 150xc2x0 C. to 400xc2x0 C. and 16 MPa to 32 MPa respectively. A more preferred combination of temperature and pressure is in the ranges of 180xc2x0 C. to 350xc2x0 C. and 20 MPa to 27 MPa respectively. At a given temperature, it is most preferred to use a pressure high enough to maintain the reaction mixture at the reaction temperature, with or without a solvent, in a fluid statexe2x80x94typically a liquid state or a supercritical fluid state. While a completely fluid state, either liquid or supercritical, is preferred, it is within the scope of the present invention that a substantially fluid state may be used. Compounds like water, CO2 or ethylene can be maintained as a supercritical fluid. The residence time is generally in the range of from 0.01 second to 20 minutes, preferably in the range of from 0.1 second to 4 minutes, more preferably in the range of from 0.5 second to 2 minutes, most preferably in the range of 1 second to 1 minute. xe2x80x9cResidence timexe2x80x9d is defined herein as the time the mixture comprising the monomers spends under the first condition for oligomerization.
A solvent or solvent mixture is not required, but may be used optionally as a medium, for the oligomerization reaction. They are herein collectively and interchangeably referred to as xe2x80x9csolvent,xe2x80x9d xe2x80x9csolventsxe2x80x9d or xe2x80x9csolvent mixture.xe2x80x9d A solvent selected for a particular oligomerization reaction should neither interfere with the desired oligomerization reaction nor react substantially with the functional group present either in any of the monomers or in the oligomer product. It is preferable that a solvent can be easily separated or removed from the reaction products by such methods as distillation, phase separation, or evaporation. If a catalyst, mediator, or initiator is used, it is preferred to have a solvent in which the catalyst or initiator is soluble in a usable amount. A mediator is a compound which, while not being able to catalyze the reaction, may nonetheless influence the reaction in a certain desirable manner. Examples of a solvent suitable for use in an oligomerization reaction include, but are not necessarily limited to, ethylene, pentane, hexane, heptane, octane, benzene, toluene, xylene(s), carbon dioxide, water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl formate, ethyl acetate, and mixtures thereof. Examples of initiators, if present, include hydrogen peroxide, alkylhydroperoxide such as t-butyl hydroperoxide and t-amyl hydroperoxide, dialkyl peroxides such as di-t-butyl peroxide, peracids, peresters, percarbonates, persulfates, ketone peroxides such as methyl ethyl ketone peroxide, oxygen, azo initiators and mixtures thereof.
In cases where the functional groups are generated by at least one post-oligomerization reaction, such a post-oligomerization reaction is carried out under a functional-group-generation condition which is known to those skilled in the art. Such post-oligomerization reactions include, but are not necessarily limited to hydrolysis, esterification, trans-esterification and epoxide ring-opening reaction. The reaction may be carried out in a solvent and/or in the presence of a catalyst. For example, in a hydrolysis, esterification, or trans-esterification reaction, an acid catalyst or a base catalyst is typically used.
The reaction between the functional group of the oligomer and the reactive moiety of a modifier is carried out under a second condition which depends on the functional group, the reactive moiety, the solvent (if present), and other physical and chemical properties of the oligomer and the modifier. The second condition comprises a temperature in the range of from 0xc2x0 C. to 450xc2x0 C. and a residence time in the range of from 0.1 second to 120 hours. Pressure is generally not a critical parameter unless the modifier has a relatively high vapor pressure at the reaction temperature. Accordingly, a wide range of pressure may be used. Ambient temperature is most convenient for most such reactions. If needed, a pressure in the range of from 1 kPa (about 0.01 bar) to 35 MPa (350 bars) maybe used. To the extent that such reactions conditions are disclosed in U.S. Pat. No. 4,059,616, U.S. Pat. No. 4,133,793, and U.S. Pat. No. 4,208,313, they are incorporated herein by reference.
This reaction between a functional group and a reactive moiety may be conveniently carried out in air if there are no substantial side reactions or by-product productions. Sometimes air or oxygen need be present in order to allow certain inhibitors such as hydroquinone to be used effectively. Optionally, a different non-reactive atmosphere may be used, particularly if air may interfere with reaction and/or cause any of the components to decompose or deteriorate. Examples of gases for providing such non-reacting atmosphere include, but are not necessarily limited to nitrogen, argon, helium or mixtures thereof. Gases like carbon dioxide also may be used alone or in conjunction with the non-reacting atmosphere described above if such gases do not interfere with the reaction and/or cause any of the components to decompose or deteriorate.
Unlike prior art products, the oligomers prepared in accordance with the process of the present invention are usually terminally unsaturated. If desired, the unsaturated terminals may be subjected to further reactions such as hydrogenation, epoxidation, or a number of other addition reactions known in the art.
A modifier suitable for the present invention depends on the nature of the functional group. They are described below in more detail. Generally, the modifier must have at least one reactive moiety which will react with the functional group. Another requirement of a suitable modifier is that it must have a crosslinkable group selected from the group consisting of carbonxe2x80x94carbon double bond (Cxe2x95x90C), an oxygen-containing heterocyclic group, and mixtures thereof, wherein the crosslinkable group remains pendant or substantially unreacted after the reaction between the modifier and the oligomer through the reactive moiety and the functional group respectively.
Examples of a reactive moiety in a suitable modifier include, but are not necessarily limited to Cxe2x80x94OH [hydroxyalkyl group], xe2x80x94C(xe2x95x90O)OH, xe2x80x94C(xe2x95x90O)OR, xe2x80x94C(xe2x95x90O)X, oxygen-containing heterocyclic group and mixtures thereof. R is selected from a C1 to C15 alkyl group or an aryl group. Examples include, but are not necessarily limited to methyl, ethyl, n-propyl, n-butyl, 2-ethylhexyl, phenyl, and mixtures thereof. X is selected from the group consisting of chloride, bromide, and iodide. Examples of an oxygen-containing heterocyclic group include oxiranyl, oxetanyl and 1,3-dioxolanyl groups of the following formula: 
R1, R2, R3, R4, R5, R6, R7, and R8 are independently selected from the group consisting of H and C1-C8 alkyl groups. H is preferred for all of the xe2x80x9cRxe2x80x9d groups. It is also preferred to have two of R1, R2 and R3 as H, and the other, CH3.
Examples of a modifier include, but are not necessarily limited to, glycidyl acrylate, glycidyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, cinnamic acid, methylcinnamic acid, acrylic acid, methacrylic acid, crotonic acid, methyl acrylate, methyl methacrylate, methyl crotonate, ethyl acrylate, ethyl methacrylate, ethyl crotonate, n-propyl acrylate, n-propyl methacrylate, n-propyl crotonate, n-butyl acrylate, n-butyl methacrylate, n-butyl crotonate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-ethylhexyl crotonate, acryloyl chloride, methacryloyl chloride, crotonyl chloride, and mixtures thereof, provided that the respective modifies are chemically compatible with each other in the mixtures.
The following reactions between the oligomer and the modifier are within the scope of the present invention whether the functional groups are present in the monomer A prior to the oligomerization or they are generated post-oligomerization from either the xe2x80x9cmonomer A equivalentxe2x80x9d or the monomer B units incorporated in the oligomer backbone structure.
I. When the functional group is hydroxy (xe2x80x94OH) groups, the reactive moieties of the modifier are selected from the group consisting of ethylenically unsaturated carboxylic acids, esters of the ethylenically unsaturated carboxylic acids, acyl halide derivatives of the ethylenically unsaturated carboxylic acids, and mixtures thereof. Examples of a monomer A in this group include, but are not necessarily limited to allyl alcohol, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxyethyl crotonate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 3-hydroxypropyl crotonate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 4-hydroxybutyl crotonate and mixtures thereof. Examples of monomer A equivalent include allyl acetate, allyl propionate, and vinyl acetate. Examples of a modifier include, but are not necessarily limited to, acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, cinnamic acid, methylcinnamic acid, methyl acrylate, methyl methacrylate, methyl crotonate, ethyl acrylate, ethyl methacrylate, ethyl crotonate, n-propyl acrylate, n-propyl methacrylate, n-propyl crotonate, i-propyl acrylate, i-propyl methacrylate, i-propyl crotonate, n-butyl acrylate, n-butyl methacrylate, n-butyl crotonate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-ethylhexyl crotonate and mixtures thereof, acryloyl chloride, methacryloyl chloride, crotonyl chloride, methacrylic anhydride and mixtures thereof.
II. When the functional group is selected from the group consisting of epoxide (oxiranyl) and carbonxe2x80x94carbon double bond; and the modifiers consist essentially of a compound selected from an ethylenically unsaturated carboxylic acid or their mixtures, and an ethylenically unsaturated alcohol or their mixtures. Examples of a monomer A in this group are 1,3-butadiene 1,2 epoxide, glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, 1-vinyl-4-cyclohexene epoxide, 1,3-butadiene, isoprene, 1-vinyl-4-cyclohexene, norbornadiene, and mixtures thereof. Examples of a modifier include acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, cinnamic acid, methylcinnamic acid and mixtures thereof. Other suitable modifiers include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxyethyl crotonate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 3-hydroxypropyl crotonate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 4-hydroxybutyl crotonate and mixtures thereof. Methylolacrylamide also may be used as a modifier.
III. When the functional group is selected from the group consisting of anhydride, alkoxysilyl, and mixtures thereof, the modifier is selected from the group consisting of hydroxyalkyl esters of ethylenically unsaturated carboxylic acids and mixtures thereof. Examples of the monomer A in this group include, but are not necessarily limited to maleic anhydride, itaconic anhydride, citraconic anhydride, xcex3-methacryloxypropyltrimethoxysilane, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltrichlorosilane, allyltriethoxysilane, allyltrichlorosilane, vinyl crotonate and mixtures thereof. Examples of modifiers include, but are not necessarily limited to 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxyethyl crotonate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 3-hydroxypropyl crotonate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 4-hydroxybutyl crotonate, and mixtures thereof.
IV. When the functional group is selected from the group consisting of hydroxyl (COH), carboxyl (COOH), amino (NH2), and substituted amino (NHR or NRxe2x80x2Rxe2x80x3) groups, the reactive moiety of a modifier consists essentially of an oxiranyl group. Examples of a preferred monomer A is selected from the group consisting of dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, and mixtures thereof. Examples of such a modifier include glycidyl acrylate, glycidyl methacrylate and glycidyl crotonate.
V. When the functional group is selected from the group consisting of an anhydride group, the reactive moiety of a suitable modifier may be an oxiranyl group. Examples of a monomer A include maleic anhydride, citraconic anhydride, itaconic anhydride, and mixtures thereof. Examples of a suitable modifier include glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, and mixtures thereof.
VI. When the functional group is selected from the group consisting of an aldehyde, a ketone group, the reactive moiety in a suitable modifier is preferred to contain a hydroxyalkyl group. Examples of the monomer A include acrolein, methacrolein, methyl vinyl ketone, and mixtures thereof. Examples of a suitable modifier include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxyethyl crotonate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 3-hydroxypropyl crotonate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 4-hydroxybutyl crotonate and mixtures thereof.
VII. When the functional group is a vinyl group as part of an ester, no reaction with a reactive moiety is needed when the curing method is selected from electromagnetic irradiations. Examples of monomer A include, but are not necessarily limited to vinyl acrylate, vinyl methacrylate, vinyl crotonate, and mixtures thereof. Examples of monomer B include, but are not necessarily limited to ethylene, propylene, C4 to C10 xcex1-olefins, butadiene, isoprene, styrene, substituted styrene such as p-methylstyrene, vinyl ester, vinyl ether, vinyl silane such as vinyltrimethylsilane, vinyl halide, acrylic acid, methacrylic acid, crotonic acid, alkyl acrylate or methacrylate, or crotonate ester such as methyl acrylate, methyl methacrylate, methyl crotonate, ethyl acrylate, ethyl methacrylate, ethyl crotonate, n-propyl acrylate, n-propyl methacrylate, n-propyl crotonate, n-butyl acrylate, n-butyl methacrylate, n-butyl crotonate, and mixtures thereof., acrylamide, methacrylamide, N-substituted acrylamide, N-substituted methacrylamide and mixtures.
VIII. When monomer A is selected from vinyl chloride, vinyl bromide, vinyl acetate, vinyl benzoate, vinylidene halide (such as chloride or fluoride) and mixtures thereof, the modifier comprises a metal salt of an unsaturated acid or a mixture of such salts. Examples of such unsaturated acids include acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, cinnamic acid, methylcinnamic acid and mixtures thereof. The metal (ion) is selected from the group consisting of metals selected from the Groups IA (Li, Na, K, Rb, Cs), IIA (Be, Mg, Ca, Sr, Ba), IIIA(Al, Ga, In, Tl) and mixtures of the period table. (see inside front cover of CRC Handbook of Chemistry and Physics, 76th Ed. 1995-1996, D. R. Lide, Editor-in-Chief CRC Press, Inc. 1995) Examples of such a salt include, but are not necessarily limited to lithium acrylate, lithium methacrylate, lithium crotonate, sodium acrylate, sodium methacrylate, sodium crotonate, potassium acrylate, potassium methacrylate, potassium crotonate, rubidium acrylate, rubidium methacrylate, rubidium crotonate, cesium acrylate, cesium methacrylate, cesium crotonate, magnesium acrylate, magnesium methacrylate, magnesium crotonate, aluminum acrylate, aluminum methacrylate and mixtures thereof. It is preferred to use phase transfer catalysis (PTC) in this case to achieve reasonable reaction rates. PTC can be effected by choosing an appropriate phase transfer catalyst(s). Depending on the catalyst selected, the amount of a phase transfer catalyst used is, based on the total moles of the modifier present, in the range of from 0 mol % to 50 mol %, preferably in the range of from 0.001 mol % to 25 mol %, most preferably in the range of from 0.01 mol % to 20 mol %.
Typical phase transfer catalysts include, but are not limited to, quaternary ammonium, phosphonium, arsonium, antimonium, bismuthonium, and tertiary sulfonium salts, crown ethers. For the salts, examples of suitable counter ions include, but are not necessarily limited to, hydroxide, halide, sulfate, bisulfate, phosphate, nitrate, and mixtures thereof. Examples of such catalysts include tetra-n-butylammonium bromide, tetra-n-butylammonium chloride, tetra-n-butylammonium iodide, tetra-n-butylammonium bisulfate, tetra-n-butylammonium hydroxide, tetraethylammonium bromide, tetramethylammonium bromide, tetra-n-propylammonium bromide, monomethyl, trioctylammonium chloride [Aliquat 336] benzyl triethylammonium bromide, hexyl triethylammonium bromide, octyl triethylammonium bromide, cetyl trimethylammonium bromide, tricaprylylmethylammonium bromide, phenyl trimethylammonium bromide, tetraphenylphosphonium bromide, triphenylmethylphosphonium bromide, tetrabutylphosphonium bromide, tetraphenylarsonium bromide, pyridyl-butyl bromide, cetylpyridinium bromide, dicyclohexano-18-crown-6 ether; 18-crown-6 and mixtures thereof. A reference in the area is Phase Transfer Catalysis: Fundamentals, Applications, and Industrial Perspectives, by C. Starks, C. Liotta, and M. Halpern, Chapman and Hall, New York, (1994). xe2x80x9cAliquatxe2x80x9d is a registered trademark of General Mill, Inc.
IX: When the functional group in monomer A is an epoxide (oxiranyl) group, examples of a monomer A include glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, monoglycidyl maleate, monoglycidyl fumarate, and mixtures thereof. Examples of monomer B include, but are not necessarily limited to, ethylene, butadiene, isoprene, styrene, p-methylstyrene, methyl acrylate, methyl methacrylate, methyl crotonate, ethyl acrylate, ethyl methacrylate, ethyl crotonate, n-propyl acrylate, n-propyl methacrylate, n-propyl crotonate, n-butyl acrylate, n-butyl methacrylate, n-butyl crotonate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-ethylhexyl crotonate, a vinyl ester such as vinyl acetate, a vinyl ether such as vinyl ethyl ether, vinyl n-propyl ether, vinyl n-butyl ether, vinyl silane such as vinyltrimethylsilane and mixtures thereof. The curing method is selected from the group consisting of acid cure, base cure, generation of acid by an electromagnetic irradiation selected from the group consisting of ultraviolet, visible light, X-ray irradiation, and xcex3irradiation and combinations thereof to produce a cured product from the radiation curable composition.
Preferred oligomer compositions (monomer A and monomer B) and the corresponding modifiers include, but are not necessarily limited to, those listed in the following Table 1:
It is also within the scope of the present invention that the oligomers, after reaction with the modifier, may be dispersed or emulsified in a solvent consisting essentially of water to form a waterborne formulation which can be used and cured, provided that there is a reasonable chemical and physical stability of the composition in such a waterborne formulation. It is generally required to have a surfactant in the formulation. Many surfactants known to those skilled in the art may be used, including but not limited to anionic surfactants, cationic surfactants, amphoteric surfactants and nonionic surfactants. Some specific examples include Triton X-100 and Triton X-200. (xe2x80x9cTritonxe2x80x9d is a registered trademark owned by Union Carbide Chemicals and Plastics Technology Corporation.)
The terms xe2x80x9ccurablexe2x80x9d and xe2x80x9ccrosslinkablexe2x80x9d are used interchangeably herein to mean that a pendant double bond or oxygen-containing heterocyclic group can be further reacted/crosslinked under a set of suitable conditions and in the presence of one parameter selected from the group consisting of a catalyst, an energy source, a free radical source, an acid, a base, or a combination thereof. The curable composition may be cured (crosslinked) by a number of methods. Examples of such methods include, but are not necessarily limited to electromagnetic irradiation such as UV irradiation(UV), visible light irradiation(VIS), xcex3irradiation, and X-ray irradiation(X-ray), electron beam irradiation (E-beam), chemical or thermal generation of free radicals, electrochemical generation of free radicals, photochemical generation of free radicals and combinations thereof. For E-beam and/or electromagnetic irradiations such as UV/VIS irradiation as the curing method(s), the curable composition may further comprise one or more photoinitiators as an additive(s) which function as free-radical initiator(s), cationic initiator(s), or anionic initiator(s). A general reference for photo free-radical generations and photoinitiators can be found in Chapter 5 of xe2x80x9cPhotogeneration Of Reactive Species For UV Curingxe2x80x9d by C. Roffey, John Wiley and Sons, New York, N.Y. (1997). To the extent the reference discloses various suitable photoinitiators and/or photo free-radical generations, it is incorporated herein by reference.
Acids or electromagnetic irradiation (such as UV and/or VIS irradiation) for generating acids or bases may be used for curing a composition having oxygen-containing heterocyclic groups such as oxiranyl, oxetanyl or 1,3-dioxolanyl groups. E-beam, UV and/or VIS irradiation(s) are three of the preferred curing methods.
The curable compositions, particularly may further comprise one or more diluent monomers as another additive, with or without one or more photoinitiators. Such a diluent monomer(s) may or may not be the same as one or more of the monomers which are already incorporated into the backbone of the oligomer(s). Many monomers or their mixtures used to form the oligomer(s) may serve the function as a xe2x80x9cdiluent monomer(s).xe2x80x9d A diluent monomer may serve to reduce viscosity, provide solvency, and/or provide additional desired properties to the final cured product, particularly for producing an electromagnetic irradiation cured product. A general reference of such diluent monomers or sometimes referred to as reactive monomers in the curing composition can be found in Chapter 6 of xe2x80x9cPhotogeneration Of Reactive Species For UV Curingxe2x80x9d by C. Roffey, John Wiley and Sons, New York, N.Y. (1997).
Table 2 provides a simplified general guideline for selecting the various components:
Generally, a UV source has a wavelength in the range of from 180 nm to 400 nm. A visible light (VIS) source has a wavelength in the range of from 400 nm to 700 nm. Information regarding EB may be found in Radiation Curing In Polymer Science and Technologyxe2x80x94Volume I, ed. J. P. Fouassier and J. F. Rabek, Elsevier Applied Science, New York, 1993 Information regarding photoacids may be found in Radiation Curing In Polymer Science and Technologyxe2x80x94Volume II, ed. J. P. Fouassier and J. F. Rabek, Elsevier Applied Science, New York, 1993 and in Prog. Polym. Sci., Vol. 21, pp 1-45, 1996.
For all the reactions involved in the process discussed herein, it is understood that they can be carried out individually in a continuous mode, a semi-continuous mode, a batch mode, a continuously stirred tank reactor mode, or a combination thereof. The various stages of the process may be carried out in the same reactor or different reactors. It is preferred to carry out the oligomerization reaction in a continuous mode. The reactor geometry and/or the residence time may be adjusted to provide different flow regimes for controlling the product yield, product composition and/or product properties. Such information is available in many references. One such reference is U.S. Pat. No. 5,710,227 (supra). Alternately, some of the reactions may be carried out simultaneously in a continuous mode, a semi-continuous mode, a batch mode, a continuously stirred tank reactor mode, or a combination thereof.
While it is generally preferred to recover the product from each individual reaction of the process prior to conducting the next reaction, the present invention also will work with minimum or no recovery or no purification. For example, it is not required to recover/separate the oligomers prior to reacting with a modifier to produce the curable compositions, or carrying out the post-oligomerization generation of functional groups. In a case where the monomers and the modifier in the feed at the same time, there is no need for any intermediate purification or separation. Typical recovery or purification methods include, but not necessarily limited to distillation, extraction, filtration, centrifugation, sedimentation, solvent removal, residual monomer removal, residual modifier removal, catalyst removal and combinations thereof.
The present invention further relates to a (curable composition prepared in accordance with the disclosed process. In particular the curable composition comprises of an oligomer having a Dp in the range of from 3 to 100 which has reacted with a modifier after the oligomer is formed, wherein the oligomer is prepared from a monomer A and monomer B. The composition may further comprise a free monomer selected from any of the monomers disclosed herein. The free monomer may or may not be the same as either the monomer A or the monomer B in the oligomer.
Preferably, the curable composition consists essentially of (a) an oligomer having a Dp in the range of from 3 to 100 which has reacted with a modifier after the oligomer is formed, wherein the oligomer is prepared from a monomer A and monomer B, (b) a free monomer selected from any of the monomers disclosed herein, (c) an initiator. The free monomer may or may not be the same as either the monomer A or the monomer B in the oligomer.
The following examples are intended for illustration purposes only. They should not be interpreted to limit the scope or spirit of the present invention which is solely defined by the claims and the specification disclosed herein.